A popular technology in the construction of silicon devices, particularly devices that include at only a few components, as contrasted with integrated circuits, is the mesa technology. This technology is characterized by the use of a mesa structure that includes one or more active junctions of the device that terminate on side walls of the mesa. Typically, a junction is formed in the mesa by diffusing an appropriate donor or acceptor impurity into a surface layer of a silicon wafer and then etching the surface to form a moat of a depth deeper than that of the diffused layer for forming a mesa that includes a diffused layer that forms a p-n junction that terminates on the side wall of the mesa. Typically, the sidewalls of the mesa where the junction terminates are later covered with one or more passivating layers, usually either of silicon nitride, an oxide of silicon, semi-insulating polysilicon, or a silicate glass.
Mesa devices that are formed in this way are usually two-terminal devices that serve a variety of functions with the area of the junction typically reflecting the use the device is to serve. Mesa diodes have been used as power rectifiers in which case the junctions have a relatively large area or as zener diodes, either unidirectional or bidirectional, or as transient voltage suppressor devices, in Which case the junctions are typically of relatively smaller area.
The mesa technology is popular because it has a number of advantages. It is a relatively simple process that results in low manufacturing costs. It is well adapted for easy manufacture of devices with a high breakdown voltage, which is largely determined by the resistivity of the wafer in which the mesa junction is formed. Additionally, mesa devices are easy to passivate.
For most of its uses, it is important that the mesa device exhibit a low series resistance when in a low resistance state. A low series resistance is advantageous to achieve a low voltage drop during forward conduction, a high clamping voltage or surge current capacity, and fast turn-on and reverse recovery speeds.
To achieve a low series resistance, it is generally important to work with relatively thin semiconductive wafers. When one works with thin wafers whose thickness is further reduced in the areas of the moats or trenches used to define the mesa, a consequence is a wafer that is prone to breaking during manufacture with resultant low yields.
It is known to increase the mechanical strength or ruggedness of mesa-type devices by providing thickened peripheral regions as is discussed in U.S. Pat. Nos. 3,553,536 and 4,220,963, but at the expense of increased complexity and cost of manufacture. The present invention represents an improved approach.